The invention relates to an optical transmitter comprising a laser diode and coupled to the laser diode a photodiode operating as a monitor diode, whose photoelectric current is used for adjusting the light power of the laser diode, and including modulators used for modulating the laser diode current by a useful signal and a pilot signal.
An optical transmitter having these characteristic features is described in the German Patent Specification DE 31 37 497 A1. The purpose of such an optical transmitter is among other things to maintain the light signal leaving the laser diode and being transferred through glass fibres at a constant level in time for virtually all parameters. These parameters may change, for example, owing to a change in environmental conditions or due to the aging process of the laser diode. The laser diode is the component which is subjected most to aging processes, whereas all the other components currently available on the market do not undergo appreciable changes during periods of time comparable to 10 or 15 years.
The aging processes affect, for example, the characteristic line of the laser diode (dependence of the radiated light power P on the current IL of the laser diode). The characteristic line for simplicity consists of two straight lines, a flat portion passing through the zero and attached thereto a steeper portion of the straight line in whose range the diode current is to be situated when the diode is in operation.
When the laser diode is in operation, the laser diode current must not fall below the common point of the two straight lines (i.e. the portions to the right and left of point SO in FIG. 1), the dip in the characteristic line, because in that case distortions will occur in the light signal of the laser. For this purpose, the laser diode bias current Iv has to be at least equal to the threshold current IO corresponding to the dip. If the position of the dip changes owing to aging processes, also the laser diode bias current is to be changed, so that the above distortions do not occur in the light signal.
On the other hand, if the slope (steepness) of the second straight portion of the characteristic line of the laser diode changes, it will change the mean radiated power of the laser diode. Since this parameter too is to be maintained at a constant level, the part of the laser diode current superposed on the bias current--the so-called modulation current--is to be readjusted.
The actual changes of the characteristic line of the laser diode consist of a superpositioning of the two parts, that is to say the shift of the dip and the reduction of the steepness. Specifically this superpositioning forms the problem with the adjustment of the transmitter power of an optical transmitter, because it is possible without any problem to determine the combined effect, but not the effect of the individual addends.
In the known arrangement the modulation current contains two control signals having the same frequency and different amplitudes, whose phases have shifted through 180.degree. relative to one another. The one control signal affects the modulation current at the top boundary and the other one at the bottom boundary. The overall laser diode current has the same effect as a laser diode current, whose bias current changes with the frequency of the control signals, as will be shown with reference to the derived formulas discussed hereinbelow.
Such a time-dependent bias current entails at least two disadvantages:
1. The light signal is full of jitter, because the time-dependent position of the edge of the light pulse depends on the fact from which value of the bias current onwards the laser diode current enhances. PA0 2. Current variations in the neighbourhood of the laser diode threshold current introduce different light frequencies into the radiated laser light, so that the optical transmission spectrum is widened. Owing to the dispersion of the transmission medium this will lead to an additional distortion of the transmitted light signal.